The present invention relates to a process for providing increased electrical and/or thermal conductivity to a material, and to the materials prepared by the process. More particularly, the invention relates to a process involving applying particles of expanded graphite to a substrate or material in order to increase the conductivity of the substrate or material. The particles of expanded graphite can be applied to the substrate or material through coating of the substrate with a composition comprising the expanded graphite particles, or by incorporating particles of expanded graphite into the substrate or material itself.
There has been a longstanding need in industry to provide electrical conductivity to non- or insufficiently conductive materials. For instance, in the automotive industry, electrostatic painting is the most highly preferred method of painting component parts. Electrostatic painting offers several times the transfer efficiency of spray painting, providing improved quality and creating significant savings by allowing paint usage to be cut by up to 75%. This has the additional benefit of greatly reducing emissions of volatile organic compounds and other hazardous pollutants, as compared with spray painting. However, electrostatic painting requires a conductive surface for effectiveness. Conventionally, when the part to be painted is a plastic or other non-metallic material, the part first has to have a primer coating of a conductive paint applied, making the electrostatic painting process more involved and therefore less desirable than otherwise.
For instance, in U.S. Pat. No. 6,019,833, Hartman, Rei, Castagnone and Hamay describe the use of a primer coating for facilitating the electrostatic painting of a non-conductive article, the primer coating containing carbon fibrils. In an attempt to address this problem, U.S. Pat. No. 6,001,919 to Yen, Ingham and Bono, describes a molding composition having a conductive filler, specifically carbon black, to form an article having sufficient conductivity to support electrostatic painting;
Likewise, certain components, such as automotive fuel system components, must be treated to dissipate static electricity, to avoid accidental ignitions. For instance, certain fuel system components are being proposed, such as fuel filler components, which are made out of plastics like nylons. Such plastic components require improved static discharge properties.
Another, unrelated area in which improved electrical conductivity may be desired is in the manufacture of printed circuit boards. Conventionally, printed circuit boards are solid circuits formed from a conductive material positioned on opposite sides of an insulating material, or in layers with insulating material interposed between the layers of conductive material. In order to make electrical connections between the circuits on the circuit board, a hole is first drilled through the board, i.e., through the conducting sheets and the interposed insulator material. These holes are known in the art as xe2x80x9cthrough holes.xe2x80x9d A conductive pathway must then be formed to connect the respective circuits. Most commonly, that conductive pathway is formed by the electrolytic deposition of copper on the surfaces of the through holes. However, the presence of insulating material makes the electrolytic deposition of copper difficult and inconsistent. As a result, sufficient conductivity must be established on the through hole surfaces to permit the electrolytic copper deposition.
Several methods have been suggested for creating sufficient conductivity to permit the consistent electrolytic deposition of copper on through hole surfaces. One such method is through the use of so-called xe2x80x9celectrolessxe2x80x9d copper, that is, copper that is chemically deposited on the through hole surfaces at a thickness sufficient to permit electrolytic deposition (but not thick enough to eliminate the need for electrolytic deposition). Although electroless deposition has proven effective, it has several commercial disadvantages. For instance, the electroless deposition process requires multiple steps prior to electroplating; involves a relatively long process time; uses multiple treatment baths; involves a complex chemistry that may require constant monitoring and individual ingredients which may require separate replenishment; uses various chemical agents that are considered carcinogens and/or are otherwise hazardous or include heavy metals, thus posing safety concerns and requiring extensive waste treatment; and utilize a multiplicity of rinse baths and thus may require large amounts of water.
In an attempt to avoid the disadvantages of the electroless deposition process, Minten and Pismannaya, in U.S. Pat. No. 4,619,741, describe coating the surfaces or walls of printed circuit board through holes with particles of carbon black, to provide sufficient conductivity to support electrolytic copper deposition. In a similar vein, Thorn, Polakovic and Mosolf, in U.S. Pat. No. 5,389,270, disclose coating the walls of printed circuit board through holes with particles of graphite having a mean particle size of from 0.05 to 50 microns. Although Thorn et al. suggest that particles of natural graphite can be used, they indicate that synthetic graphite is preferred.
Prior methods disclosed for providing conductivity to insufficiently conductive articles suffers from significant drawbacks. For instance, carbon fibers, carbon fibrils, nanotubes, nickel coated carbon fibers, steel fibers, carbon blacks, etc. have been proposed as conductive fillers, but the loading levels required in many prior art methods to provide the required degree of conductivity can be prohibitively high; likewise, consistency is often a problem, as is weight and density. Similarly, the cost of some of the prior art methods, such as carbon fibrils or nanotubes, can also be prohibitive.
Other relatively non-conductive materials, including greases and oils, can act as heat and electrical insulators, and thereby do not sufficiently dissipate heat and static electricity from, e.g., gear boxes and other moving metallic parts where friction can generate heat and static electricity. Failure to sufficiently dissipate the heat and static electricity can cause undue or early corrosion.
There are also many applications in which conductive adhesives may be desirable. For instance, adhesives are often used to bond materials such as heat sinks or thermal interfaces to a heat source. With insufficient conductivity, an adhesive can interfere with the desired thermal transfer, degrading the usefulness of the heat sink or thermal interface. In addition, a conductive adhesive can also function to facilitate grounding of an electrical device in which it is used.
What is desired, therefore, is a process for increasing the conductivity of an insufficiently conductive material, article or surface, which does not adversely affect the other desirable properties of the material, article or surface. Such a process is cost-effective, and provides sufficient conductivity without undesirably high loading levels or weight increase.
It is an object of the present invention to provide a process for increasing the conductivity of an insufficiently conductive material, article or surface.
It is another object of the invention to provide a process that increases the conductivity of a material, article or surface while preserving the desirable characteristics of the material, article or surface.
It is yet another object of the invention to provide an article having sufficient conductivity to permit the electrostatic painting of the article.
It is still another object of the present invention to provide a printed circuit board having through hole walls sufficiently conductive to permit the electrolytic deposition of copper on the walls.
It is a further object of the present invention to provide an oil or grease sufficiently conductive to at least partially dissipate the heat and/or static electricity generated by moving parts.
It is still another object of the present invention to provide an adhesive which is sufficiently conductive to minimize the reduction of thermal or electrical conduction between two object bonded by use of the adhesive.
These objects and others that will become apparent to the artisan upon review of the following description can be accomplished by a process for providing increased electrical and/or thermal conductivity to a material or a substrate, especially a plastic substrate. Suitable substrates include an automobile component part or a printed circuit board. The process involves coating at least some of the surfaces of the substrate with a composition that includes particles of expanded graphite. Preferably, the particles of expanded graphite are formed into a powder prior to coating the composition on the substrate, and the composition further includes a carrier for the powdered particles of expanded graphite, the carrier being a material capable of forming an adherent coating on the targeted surfaces of the substrate.
In another embodiment of the invention, a process for providing increased electrical conductivity to a substrate is provided, where the process involves incorporating particles of expanded graphite into the substrate. As noted, the process preferably utilizes the particles of expanded graphite formed into a powder prior to incorporating the expanded graphite into the substrate.
In alternative embodiments of the invention, the particles of expanded graphite are incorporated into a grease or oil such as a polyolefin or other hydrocarbon grease or oil; an adhesive such as a acrylic, starch, polyethylene or epoxy adhesive composition; or a rubber composition, to provide increased thermal and/or electrical conductivity to the material into which the particles of expanded graphite are incorporated.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. Graphites possess anisotropic structures and thus exhibit or possess many properties such as electrical conductivity that are highly directional. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two sets of axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers (parallel to the planar direction of the crystal structure of the graphite) or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded graphite structure (also referred to as exfoliated or intumesced graphite) in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated articles and flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension into integrated articles and flexible sheets by compression, without the use of any binding material, is believed to be possible due to the excellent mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
Generally, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is at least about 80 times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. Controlling the degree of compression can vary the density and thickness of the sheet material. The density of the sheet material can be within the range of from about 0.1 grams per cubic centimeter to about 1.5 grams per cubic centimeter.
Once the flexible graphite sheet is prepared, it can be formed into a powder by, for instance, air classifier milling, jet milling, ball milling, hammer milling or by other processes which would be familiar to the artisan. Alternatively, the particles of expanded graphite, prior to being formed into a flexible graphite sheet, can be formed into a powder for use in the present invention.